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A team of scientists has found bacteria living nearly two miles below ground, dining on sulfur in a world of steaming water and radioactive rock. A single cell may live a century before it gets up the energy to divide. The organisms have been there for millions of years. They will probably survive as long as the planet does, drawing energy from the stygian world around them.

The microbes, found in water spilling out of a fissure in a South African gold mine in 2003, are not entirely new, the researchers report in today's issue of Science. They are similar to ones found in other extreme environments and among the most primitive life forms ever described.

What is unusual is that their underground home contains no nutrients traceable to photosynthesis, the sunlight-harnessing process that fuels all life on Earth's surface. Such a community is an oddity on this planet -- and is of interest to people looking for life on other ones.

"There is an organism that dominates that environment by feeding off an essentially inexhaustible source of energy -- radiation," said Tullis C. Onstott, a geoscientist at Princeton University who led the team. "The bottom line is: Water plus rocks plus radiation is enough to sustain life for millennia."

The surfaces of other rocky bodies in the solar system are all too cold, too hot, too dry or too toxic to support the kind of life known on Earth. Their subterranean environments, however, are likely to be more hospitable and stable. More important, many may contain the short list of ingredients that seem to be all the South African microbes need.

"This is a very nice potential model of the habitability of Mars, Jupiter's Europa and other moons," said Steven D'Hondt, an astrobiologist at the University of Rhode Island, who was not involved in the project. "The sorts of ecosystems you could get there could certainly be something like this."

Onstott agreed.

Mars is known to have both subsurface water and uranium. Onstott's team's findings suggest that even without volcanoes to warm the Martian environment, organisms that may have evolved in a more temperate time may survive there.

"The existence of radiation may be enough to keep life going, and perhaps even thriving and evolving," he said. "I think this really increases the likelihood that we will find life beneath the surface of Mars."

For more than two decades, microbiologists have been able to find and retrieve permanent colonies of bacteria living hundreds or even thousands of feet below ground. In virtually all cases, however, the subterranean environments contained carbon-based molecules from decayed plants or animals. The energy in those molecules' chemical bonds was all traceable to the sun, captured by plants through photosynthesis.

The microbes from the South African mine appear to exist outside this food chain. The underground chemistry appears to go like this: